Thrust control mechanism

ABSTRACT

A thrust control mechanism for use in association with a combustion engine having a valve means to control the flow of fuel to said combustion engine upon command, which is responsive to a pressure responsive member for sensing thrust as a function of the pressure drop across said combustion engine exhaust nozzle. Said thrust control mechanism may further include an absolute reference means to compensate for changes in ambient pressure of the device and a variable nozzle area input means to compensate for changes in combustion engine thrust due to different settings of nozzle area.

Unite States Patent 91 Arnett [S4] THRUST CONTROL MECHANISM [75]Inventor: Samuel E. Arnett, South Bend, Ind.

[73] Assignee: The Bendix Corporation, South Bend, Ind.

[22] Filed: Nov. '18, 1970 [21] Appl. No.: 90,777

Related U.S. Application Data [62] Division of Ser. No. 748,416, July29, 1968, Pat. No

9/1968 Urban 60/242X 3/1969 Kast ..60/242 Primary Examiner-Clarence R.Gordon Attorney-Gordon H. Chenez et a1.

[ 5 7] ABSTRACT A thrust control mechanism for use in association with acombustion engine having a valve means to control the flow of fuel tosaid combustion engine upon command, which is responsive to a pressureresponsive member for sensing thrust as a function of the pres sure dropacross said combustion engine exhaust nozzle. Said thrust controlmechanism may further include an absolute reference means to compensatefor changes in ambient pressure of the device and a variable nozzle areainput means to compensate for changes in combustion engine thrust due todifferent settings of nozzle area.

4 Claims, 4 Drawing Figures 2 Sheets-Sheet 2 my Q FIGZ

BACKGROUND OF THE INVENTION This invention relates, in general, tocombustion engine fuel controls and, in particular, a thrust controlmechanism to override upon command a conventional fuel control conceptfor a gas turbine engine.

The conventional gas turbine engine fuel control systems with which I amfamiliar employ concepts and structure that maintain engine speed at aselected value. Thus, for example, as a vertical take-off and landingaircraft approaches a landing surface, hot exhaust gases are deflectedoff the landing surface and recirculate to the gas turbine engine inlet.This increases the inlet temperature and engine speed starts to increasebeyond the selected value. To decrease speed, the fuel fiow to theengine is then automatically reduced with a resulting decrease inthrust. This sequence of events could result in a serious aircraftattitude problem and an unstable uncontrolled landing. Further, someaircraft experience during supersonic flight, a power setting of enginespeed or afterburner fuel-to-air ratio which result in an unstableintersection of thrust and drag. Still further, specific selection ofspeeds on a multi-engine aircraft does not generally result in optimizedindividual engine performance in terms of fuel consumption.

SUM MARY OF THE INVENTION It is the purpose of this invention to providea thrust control mechanism whereby engine performance may be controlledas a function of thrust measured across said gas turbine engine exhaustnozzle. With regard to vertical take-off and landing, if thrust in lieuof engine speed were being controlled, the engine control would changeonly enough to compensate for changes in specific fuel consumption. Thatis to say, since thrust is equal to the fuel flow divided by specificfuel consumption, the only change that is required is to compensateforthe change in specifics. This change is small in magnitude and occurssmoothly resulting in improved landing control.

It is another object of this invention such that at the unstableintersection of thrust and drag that can exist in supersonic flight,thrust control of the gas turbine engine may be selected and,automatically, a stable thrust-drag intersection would result.

It is anotherobject of this invention to provide individual thrustcontrol of the various engines so that the thrust may be varied alongwith aircraft trim to obtain optimum gas turbine engine fuelconsumption.

Other objects and features of the invention will be apparent from thefollowing description of the thrust control mechanism taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional schematic of thecomponents comprising the thrust control mechanism shown in associationwith a gas turbine engine and its conventional fuel control;

FIG. 2 is a modified sectional schematic of the component; comprisingthe thrust control mechanism shown in FIG. 1 but excluding the resilientmeans for providing rate of change of fuel flow information.

FIG. 3 is a modified sectional schematic of the components comprisingthe thrust control mechanism shown in FIG. 1 including an absolutereference means; and

FIG. 4 is a modified sectional schematic of the components comprisingthe thrust control mechanism shown in FIG. 3 including a variable nozzlearea input means.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand particularly to FIG. 1, numeral 10 designates a gas turbine enginehaving a casing 12 with an air inlet 14 and an outlet or exhaust nozzle16. Air from the inlet 14 flows through a compressor 18 driven by aturbine or turbines 20 via a shaft 22 suitably mounted for rotation incasing 12. The pressurized air at pressure P, discharged from compressor18 flows to combustion chambers 24 where pressurized fuel injected byfuel nozzles 26 is mixed with the air and burned to provide a flow ofhot motive gas. The hot motive gas flows through turbine 20 driving thesame and exhausts therefrom to the nozzle 16 from which the gas exits tothe atmosphere to provide propelling thrust.

The fuel nozzles 26 are connected to a fuel manifold 28 which receives apressurized metered flow of fuel via a fuel conduit 30 leading from theoutlet 32 of a fuel control 34. A supply fuel conduit 36 connects a fueltank 38 with inlet 40 of fuel control 34. An engine driven fuel pump 42connected to conduit 36 serves to pressurize the fuel passing to inlet40.

The fuel control 34 is provided with a casing 44 having inlet and outletports, 40 and 32, respectively, formed therein. Fuel flow from the inlet40 to the outlet 32 is controlled by a valve means comprising a variablearea metering orifice 46 and a metering or control valve 48 whichcooperates with the orifice 46 to establish an effective flow area, andthus, rate of fuel flow to the combustion chamber 24. A constantpredetermined fuel pressure drop P P across orifice 46, is maintained bya conventional by-pass valve mechanism generally indicated by 50 whichis responsive to the pressure drop P P across orifice 46 and whichfunctions to divert more or less fuel at pressure P depending upon therelative pressure drop error across orifice 46 from the upstream side oforifice 46 back to the inlet of fuel pump 42 at relatively low pressureP to thereby cause a decrease or increase in the pressure as required tomaintain the constant predetermined drop P P A drain passage 52 ventsthe interior of casing 44 to passage 36 at fuel pump inlet pressure P,,.

The metering valve 48 is operatively connected to and actuated by aservo piston 54 slidably carried in casing 44 and separating chambers 56and 58 which are pressurized by fuel controlled by a servo flapper valve60 in cooperation with orifice 62 which receives fuel pressure P througha restriction 64 from fuel pressure P Chamber 56 receives said fuelpressure P through passage 65 as a function of the position of flappervalve 60 relative to orifice 62. The servo piston 54 is operativelyconnected to a fuel scheduling cam 66, suitably guided on one end by thecasing 44 and slidably retained in a cavity 68 of the casing 44 on itsother end,

by means of a spring retainer 70 operatively attached to said servopiston 54, a spring 72, a double-ended spring retainer 74, a spring 76and a spring retainer 77 slidingly retained by casing 44. Servo flappervalve 60 is pivotally retained by casing 44 via pin 78 and pivotallyattached intermediate said doubleended spring retainer by pin 80. Thefuel scheduling cam 66 may be operatively connected to reflect numerousparameters as variable functions of engine performance or pilotinitiated commands to the engine. The spring 76 serves to convert theposition setting of the fuel scheduling cam 66 into a force acting uponthe servo flapper valve 60 and the spring 72 reacts against its retainer70, being opposed to movement by fuel pressure P,, to provide aforce-opposing feedback which nulls the force transmitted to the servoflapper valve 60 by the spring 76. The servo flapper valve 60 isresponsive to the fuel scheduling cam 66 so as to control the effectivearea of orifice 62 and thus regulate the servo pressure P in chamber 56to position servo piston 54.

The servo flapper valve 60 in cooperation with orifice 62 derive adownstream pressure P that may be made available at an outlet 61 tofacilitate further control of the fuel control 34 by a complementingengine control system. Further, an inlet 63 is provided in the casing 44to vent returning fuel from the complementing engine control system topressure P,,. As may be seen by those skilled in the art, the fuelcontrol 34 and the complementing engine control may be combined into asingle structure should it be desirable for a specific application. If acomplementing engine control is not used, the outlet 61 may merely bevented to pressure P through inlet 63 to make an operable system.Chamber 58 has a pressure P that will vary in proportion to the rate ofchange of the position of the servo piston 54. Pressure P may be madeavailable at outlet 82 to provide a lead function to a complementingengine control system. Pressure P is vented through a restriction 84 tofuel pressure P,,. Thus, during any period when the servo piston 54 isnot changing position, pressure P will equal P,, and a lead functionsignal will not be reflected by pressure P at outlet 82. If acomplementing engine control system is not used the outlet 82 may merelybe closed off to prevent fuel leakage to the outside ambient.

A thrust control mechanism 86 complements the fuel control 34 forcontrol of the gas turbine engine 10. The thrust control mechanism 86,as shown in FIG. 1, has a casing 88 wherein a diaphragm 90 defineschambers 92 and 94. Chamber 92 receives total nozzle pressure P throughinlet 96, passage 98 and pressure sensing element 100, immediatelyupstream from the gas turbine exhaust nozzle 16. The diaphragm 90 isresponsive on one side to variations in nozzle pressure P,. Diaphragm 90is responsive on its other side to ambient air pressure P enteringchamber 94 through inlet 102 and has rigidly attached thereto a shaft103 and a spring retainer 104. A second shaft 105 and spring retainer106 are slidingly retained by casing 88. A spring 107 is retainedbetween the spring retainers 104 and 106 and therewith comprises athrust request means which translates thrust position information into aforce to act upon a valve or lever means 108. The diaphragm 90 isresponsive to the thrust of the gas turbine engine and acts in aforce-opposing relationship with the thrust request means to null anyrequested increase or decrease in thrust. The lever means 108 ispivotally attached to the casing 88 by pin 110 and has its one endpivotally attached to the shaft 103 by pin 112 while its other endextends into chamber 1 14 and defines a valve 1 16. Chamber 1 14receives fuel pressure P, from outlet 61 through passage 117, inlet 118and an orifice 120 and vents through outlet 122, passage 124 and inlet63 to fuel pressure P A bellows 126, responsive through passage 128 andinlet 130 to servo piston rate of change pressure P is rigidly fixed onone end to casing 88 and pivotally attached by pin 132 to lever means108. The bellows 126 relates to the lever means 108, an anticipatoryforce as a function of the rate of change of the servo piston 54 and thecorresponding fuel flow to the gas turbine engine 10 to damp theresponse of the lever means 108 to the request for thrust means. Upon arequest for thrust the valve 116 in cooperation with orifice 120controls the pressure P,, and can thereby override the normal control ofthe fuel control 34 by the fuel scheduling cam 66 to decrease the fuelflow rate established by fuel scheduling cam 66. It is noted that thethrust control mechanism 86 may never override the fuel scheduling cam66 to increase the fuel flow rate established by the fuel scheduling cam66. When the thrust control mechanism 86 overrides the otherwise normalfunctioning of the fuel control 34, fuel flow to the gas turbine engine10 will vary directly as a function of a specific request for thrust. Itis noted that the shaft 105, responsive to a request for thrust, may beoperatively attached to a manual control system available to theoperator of the gas turbine engine 10 or preprogrammable by structuresimilar to fuel scheduling cam 66 or equivalent.

As can be seen by those skilled in the art the fuel control 34 andthrust control mechanism 86 casings can be made of two or more parts toenable assembly of the various components therein. Although notpreviously mentioned, those skilled in the art will further see thatseals may be employed in the appropriate places to preclude fuelleakage.

MODE OF OPERATlON OF THE PREFERRED EMBODIMENT Initially, it will beassumed that the gas turbine engine 10 is stable in operation with thefuel scheduling cam 66 set to provide a near maximum fuel flow. Thus,the flapper valve 60 is open relative to orifice 62. Further, it isassumed that a reasonably low value of thrust has been selected, andthus, the flapper valve 116 is only partially opened with respect toorifice 120. The system as a whole is in a null condition.

The shaft is moved in the direction of the arrow shown in FIG. 1, torequest increased thrust from the gas turbine engine 10. Spring 107 willcompress and convert a position input from shaft 105 and spring retainer106 into a force, causing spring retainer 104 and shaft 103 to actagainst the diaphragm 90 and nozzle pressure P, such that lever means108 is pivoted about pin to further open valve 116 relative to orifice120. Thus, pressures P and P dro n, since valve 60 is open with respectto orifice 62. The drop in pressure P allows servo piston 54 to move tothe left in response to pressure P acting against valve 48. As servopiston "54 and valve 48 move to the left, increased fuel flow isprovided to the gas turbine engine which upon combustion results inincreased thrust venting through exhaust nozzle 16. The increase inthrust has a finite response time which is a function of the flow ratesof the system, and thus, increased thrust is not instantaneouslyavailable upon a request for increased thrust. The increasing thrustwill be sensed by element 100 and communicated through passage 98 andinlet 96 to chamber 92 to act upon diaphragm 90 in opposition to therequest for increased thrust. The increased nozzle pressure P, will,acting alone on diaphragm 90, eventually null the thrust controlmechanism 86 and thus the fuel control 34. However, as servo piston 54moves to the left in response to a request for increased thrust, springretainer 70 compresses spring 72 so as to exert a first force on flappervalve 60 to further close it relative to orifice 62. This first force isopposed by a second force resulting from spring retainer 74 tending tocompress spring 76 against spring retainer 77. The second force tends todamp the first force with respect to further closing flapper valve 60and bringing the system to a null. To further refine the responsivenessof the system, a thrust request lead signal may be provided toanticipate the conditions necessary for a null as soon aspossible aftera thrust request has been made. The fuel control 34 derives such a leadsignal at outlet 82 as servo piston 54 moves to the left in response toa request for increased thrust. The existing pressure P in chamber 58will be equal to pressure P however, as piston 54 moves to the left thevolume of chamber 58 will increase and the pressure P will drop belowpressure P since restriction 84 will preclude instantaneous flow of P,into chamber 58. The actual pressure level of pressure I will vary as afunction of the rate of change of position of servo piston 54. Thus, thefaster the rate of change of position of piston 54 the greater thepressure difference between P and P since a slow rate of change wouldallow P, to flow through restriction 84 into chamber 58 so as topreclude a pressure difference. The P pressure drop developed in chamber58 will be communicated via outlet 82, passage 128 and inlet 130 tobellows 126. The bellows 126 will retract tending to cause lever means108 to further close valve 116 relative to orifice 120. This lead signalis functionally intended to dampen and smooth the responsiveness of thethrust control mechanism 86 to the increasing nozzle pressure P as it isimposed upon the diaphragm 90, and thus lever means 108 to close valve116 relative to orifice 120 in bringing the thrust control mechanismagain to a null condition. As valve 116 approaches a null condition withrespect to orifice 120, pressures P and P will increase. When pressureP, increases, it will cause the servo piston to approach a nullcondition relative to pressure P acting on valve 48. Thus, when thethrust control mechanism 86 nulls, the fuel control 34 willcorrespondingly null.

It is understood that a request to the thrust control mechanism todecrease thrust will result in an identical converse action.

DESCRIPTION OF THE MODIFIED EMBODIMENTS In the embodiments shown inFIGS. 2, 3, and 4, those parts which are identical to correspondingparts of the preferred embodiment, depicted in FIG. 1, will be given thesame identifying numbers.

The thrust control mechanism shown in FIGS. 2, 3, and 4, are intended tofunction in cooperation with the fuel control 34 shown in FIG. 1 forcontrol of the gas turbine engine 10.

It is parenthetically mentioned that the thrust control mechanism 86shown in FIG. 1 need not contain the bellows 126 to provide thrustcontrol override of the fuel control 34. The bellows 126 enables thrustcontrol refinement in that it provides an input to the thrust controlmechanism to allow it to more smoothly achieve a null condition inresponse to a request for a change in thrust. Thus, FIG. 2 depicts asectional schematic of the components comprising the thrust controlmechanism shown in FIG. 1, but excluding the bellows 126 to depict asimplified form of the invention.

FIG. 3 is a modified sectional schematic of the components comprisingthe thrust control mechanism shown in FIG. 1 including an absolutereference means 140. The absolute reference means is comprised of anevacuated bellows 142 being rigidly mounted to casing 88 on one end andfree on its other end to vary as a function of ambient pressure changes,a shaft 144 pivotally attached by a pin 146 to the bellows 142, and aroller means 148 pivotally attached by a pin 150 to the shaft 144. Theroller means 148 is operatively positioned intermediate the lever means108 and the spring retainer 104 to transmit a request for thrust forcefrom said spring retainer 104 to said lever means 108. The position ofthe roller means 148 is a function of bellows 142 expansion orcontraction in response to a decreased or increased ambient pressure PThus, as the roller means 148 changes position, the lever arm of thethrust request force is changed to compensate for variations in ambientpressure P FIG. 4 is a modified sectional schematic of the componentscomprising the thrust control mechanism shown in FIG. 3 including avariable nozzle area input means 152. The variable nozzle area inputmeans 152 is comprised of a lever 154 pivotally attached to casing 88 bya pin 156, ashaft 158 slidingly projecting outside the casing 88 throughan opening 159, and a roller means 160 pivotally attached by a pin 162to the shaft 158. The lever 154 operatively engages the roller means160. The roller means 160 and the lever 154 are operatively positionedintermediate the lever means 108 and the roller means 148 to transmit arequest for thrust force from said spring retainer 104 and roller means148 to said lever means 108. The position of the roller means 160 is afunction of the response of shaft 158 to a change in engine exhaust l6nozzle area through a linkage (not shown) from the gas turbine engine 10to the thrust control mechanism 86. Thus, as the roller means 160changes position, the lever arm of the thrust request force is changedto compensate for changes in engine exhaust nozzle 16 area.

MODE OF OPERATION OF MODIFIED EMBODIMENTS With reference to FIG. 3, thethrust control mechanism 86 is provided an absolute reference means 140to compensate for ambient pressure P changes experienced while operatingthe gas turbine engine 10 at varying altitudes. It is assumed thatthegas turbine en gine 10, the fuel control 34, and the thrust controlmechanism 86 are mounted on an aircraft, not shown. As the aircraftclimbs in altitude the ambient pressure I, decreases, and a greatervalue of I P, is necessary to maintain the predetermined thrust request.Bellows 142 will expand and position the roller means 148 to the left togive the request for thrust force a larger lever arm with respect tolever means 108 thus requiring an increase in P, P to null the system.

It is noted that an absolute reference means may or I may not be adesirable compensation forthe thrust control mechanism depending uponthe responsiveness of the control to sensing errors resulting fromaltitude changes.

It is understood that a decrease in altitude causing an increase inambient pressure P will result in an identical converse action.

With reference to FIG. 4, the thrust control mechanism 86 is furtherprovided a variable nozzle area input means 152 to compensate forchanges in engine thrust as a function of the area of the exhaust nozzleof gas turbine engine 10. It is assumed that the gas turbine engine 10,the fuel control 34 and the thrust control mechanism 86 are mounted onan aircraft, (not shown) and that the engine is being operated undernormal conditions. If the operator of the aircraft makes a changethrough a conventional nozzle area varying mechanism, (not shown), toreduce the area of engine exhaust nozzle 16, a change in P, P must berequested to maintain the same predetermined thrust as set by shaft 105.As nozzle area reduces the linkage responsive to engine exhaust nozzle16 area and operatively attached to shaft 158 will position the rollermeans 160 to the left to give the request for thrust force a largerlever arm with respect to lever means 108, thus P P must increase toexert a nulling force on the lever means 108 due to the decreasingchange in nozzle 16 area. I

It is understood that a change in controls to increase the area ofengine exhaust nozzle 16 will result in an identical converse action.

While the specific details have been herein shown and described, theinvention is not confined thereto, as other substitutions can be madewithin the spirit and scope of the invention.

1 claim:

1. A thrust control mechanism for use with a combustion engine equippedwith an exhaust nozzle and comprising:

a valve means operative to control the flow of fuel to said combustionengine;

a means responsive to a request for increased or decreased thrust;

said valve means being responsive to said means responsive to a requestfor increased or decreased thrust to accordingly increase or decreaserespectively, the fuel flow to said combustion engine and therebyincrease or decrease engine thrust;

a pressure responsive member receiving exhaust nozzle pressure on oneside and ambient pressure on its other side for sensing thrust as afunction of the pressure drop across said exhaust nozzle;

said valve means being responsive to said pressure responsive member;

said pressure responsive member being in a forceopposingrelationshipwith said means responsive to a request for thrust andresponsive to a change in engine thrust to move said valve means to nullsaid thrust control mechanism;

an evacuated bellows mounted to rigid structure on one end and free onits other end to vary as a function of ambient pressure changes;

a shaft pivotally attached to said other end of said evacuated bellowsand having a roller means pivotally attached to its other end;

said roller means being operative intermediate said means responsive toa request for thrust and said valve means to transmit force from saidmeans responsive to a request for thrust to said valve means whilecompensating for change in ambient pressure in response to saidevacuated bellows variations.

2. A thrust control mechanism for use with a combustion engine asrecited in claim 1 including a variable nozzle area input meanscomprising:

an input shaft variable as a function of a combustion engine outletnozzle area;

a second roller means pivotally attached to said input shaft;

said second roller means being operative intermediate said meansresponsive to a request for thrust and said valve means to transmitforce from said means responsive to a request for thrust to said valvemeans while compensating for changes in nozzle area in response to saidinput shaft variation.

3. A thrust control mechanism for use with a combustion engine asrecited in claim 1 wherein:

said means responsive to a request for increased or decreased thrustincludes a resilient member having an output force representing a thrustrequest;

a lever pivotally mounted for movement on a fixed axis and operativelyconnected to said valve means for actuating the same;

saidroller means interposed between said resilient means and said leverand movable relative to said fixed axis to vary the effective lever, armof said lever in response to said evacuated bellows.

4. A thrust control mechanism for use with a combustion engine asclaimed in claim 2 wherein:

said means responsive to a request for increased or decreased thrustincludes a resilient member having an output force representing a thrustrequest;

a first lever pivotally mounted for movement on a fixed axis;

said first named roller means being interposed between said resilientmeans and said lever and movable relative to said fixed axis to vary theeffective lever arm of said lever;

a second lever pivotally mounted for movement on a fixed axis andoperatively connected to said valve means for actuating the same;

said second roller means being interposed between said first lever andsaid second lever and movable relative to said fixed axis of said firstlever to vary the associated effective lever arm thereof.

1. A thrust control mechanism for use with a combustion engine equippedwith an exhaust nozzle and comprising: a valve means operative tocontrol the flow of Fuel to said combustion engine; a means responsiveto a request for increased or decreased thrust; said valve means beingresponsive to said means responsive to a request for increased ordecreased thrust to accordingly increase or decrease respectively, thefuel flow to said combustion engine and thereby increase or decreaseengine thrust; a pressure responsive member receiving exhaust nozzlepressure on one side and ambient pressure on its other side for sensingthrust as a function of the pressure drop across said exhaust nozzle;said valve means being responsive to said pressure responsive member;said pressure responsive member being in a force-opposing relationshipwith said means responsive to a request for thrust and responsive to achange in engine thrust to move said valve means to null said thrustcontrol mechanism; an evacuated bellows mounted to rigid structure onone end and free on its other end to vary as a function of ambientpressure changes; a shaft pivotally attached to said other end of saidevacuated bellows and having a roller means pivotally attached to itsother end; said roller means being operative intermediate said meansresponsive to a request for thrust and said valve means to transmitforce from said means responsive to a request for thrust to said valvemeans while compensating for change in ambient pressure in response tosaid evacuated bellows variations.
 2. A thrust control mechanism for usewith a combustion engine as recited in claim 1 including a variablenozzle area input means comprising: an input shaft variable as afunction of a combustion engine outlet nozzle area; a second rollermeans pivotally attached to said input shaft; said second roller meansbeing operative intermediate said means responsive to a request forthrust and said valve means to transmit force from said means responsiveto a request for thrust to said valve means while compensating forchanges in nozzle area in response to said input shaft variation.
 3. Athrust control mechanism for use with a combustion engine as recited inclaim 1 wherein: said means responsive to a request for increased ordecreased thrust includes a resilient member having an output forcerepresenting a thrust request; a lever pivotally mounted for movement ona fixed axis and operatively connected to said valve means for actuatingthe same; said roller means interposed between said resilient means andsaid lever and movable relative to said fixed axis to vary the effectivelever arm of said lever in response to said evacuated bellows.
 4. Athrust control mechanism for use with a combustion engine as claimed inclaim 2 wherein: said means responsive to a request for increased ordecreased thrust includes a resilient member having an output forcerepresenting a thrust request; a first lever pivotally mounted formovement on a fixed axis; said first named roller means being interposedbetween said resilient means and said lever and movable relative to saidfixed axis to vary the effective lever arm of said lever; a second leverpivotally mounted for movement on a fixed axis and operatively connectedto said valve means for actuating the same; said second roller meansbeing interposed between said first lever and said second lever andmovable relative to said fixed axis of said first lever to vary theassociated effective lever arm thereof.